The present invention generally relates to wireless communication systems, such as wireless Local Area Networks (LANs). Typically, wireless LAN systems are placed within offices or homes to allow for wireless communication with remote units. For example, hand-held computers can be wirelessly connected to other computers, printers, and the like. The present invention also relates to outdoor wireless communication systems. Outdoor wireless communication systems are used, for example, to provide connectivity between two or more physically separated independent Local Area Networks. Wireless communication systems can also provide remote access to a networking infrastructure where wired connectivity is impractical or cost-prohibitive.
In some wireless communication systems, signals from multiple paths are received. When the data rate is low, this does not cause a significant problem. However, at high data rates, the multiple paths can cause high levels of intersymbol interference (ISI).
Another issue in wireless communication systems is data rate versus bandwidth. In frequency-hopping wireless communication systems, the bandwidth available at each hop is restricted. In the U.S., FCC regulations strictly limit transmissions out of this bandwidth.
In order to get a relatively high data rate within a relatively small bandwidth, complex modulation schemes such as QAM have been used. QAM is one of a class of linear modulation techniques in which information is conveyed in the amplitude as well as the phase of the carrier. Typically these modulation schemes require linear transmitters or receivers. These modulation schemes have the disadvantage of increasing the cost of the receivers and transmitters as well as reducing the power efficiency of the transmitters. This is especially undesirable for wireless communication systems that need to use inexpensive units or are used in battery-powered devices.
A non-linear modulation technique that has been used in the past to address the need to reduce the transmission bandwidth of the signal for a given data rate is Gaussian Frequency Shift Keying (GFSK). This modulation scheme is inexpensive and has good power efficiency. In order to reduce the required bandwidth for a given data rate, the modulation index (defined as the frequency deviation used, divided by the bit rate) can be reduced to between 0.15 and 0.2. However, performance of the receiver suffers greatly at these levels. In more common implementations, the modulation index is kept at 0.5. At a 0.5 modulation index, this form of modulation is called GMSK and is widely used in cellular telephony.
Another type of system attempting to fit a higher bit rate through a fixed bandwidth are systems called partial response systems. Here, a predetermined amount of ISI is inserted into the modulation at the transmitter, and it is later corrected by the receiver. The correction does not deal with channel impairments such as multipath (which creates additional ISI) but simply with the ISI introduced by the transmitter.
Finally, techniques for dealing with channel-induced ISI have been widely used. These systems require the receiver to be a linear receiver to ensure that the integrity of the received signal is not compromised. For example, limiting receivers are a type of non-linear receiver where the amplitude information of the incoming signal is lost. However, limiting receivers are generally less expensive than linear receivers.
It is desired to have an improved wireless communication system.
The present invention is a frequency-hopping wireless communication system with a high data-rate over hop-bandwidth ratio, a relatively simple modulation scheme, and the ability to operate with the relatively high levels of intersymbol interference of a high data-rate multipath environment.
The transmitter preferably overfilters a binary or quaternary frequency shift keying (BFSK or QFSK) transmitted signal. This improves the data-rate over hop-bandwidth ratio of the QFSK transmitted signal but results in transmitter-induced intersymbol interference. In a preferred embodiment, the receiver deals with the induced intersymbol interference.
The receiver preferably uses a demodulator with a maximum likelihood sequence detector that can deal with high levels of intersymbol interference of the high data-rate multipath environment. In a preferred embodiment, the maximum likelihood sequence detector uses data from multiple symbol periods in its sequence determination. A channel impulse response estimation is preferably done to get an indication of the multipath transmissions.
One embodiment of the present invention is a frequency-hopping wireless communication system including a receiver adapted to receive data at a relatively high data rate. The received signals being quaternary frequency shift keying signals limited to a relatively low bandwidth at each frequency hop. The ratio of the data rate over the xe2x88x9220 dBc bandwidth of the received signal at each frequency hop is preferably greater than two Mbps/MHz. The receiver is able to operate in a multipath environment.
Another embodiment of the present invention is a frequency shift keying receiver comprising analog elements including a down-converter unit and limiter; and digital elements including a slope compensation filter, a digital gain control unit, a frequency discriminator unit and a frequency shift keying demodulator.
Yet another embodiment of the present invention is a frequency shift keying receiver comprising analog elements including a down-converter unit and a limiter; a slope compensation filter, the slope compensation filter compensating for frequency distortion induced by the analog elements; and a frequency discriminator unit after the slope compensation filter.
Another embodiment of the present invention is a receiver adapted to receive data at first and second data rates, the first data rate being associated with quaternary frequency shift keying signals, and the second data rate being associated with binary frequency shift keying signals. The binary and quaternary frequency shift keying signals having the same symbol rate. The receiver including a maximum likelihood sequence detector unit implementing trellis state diagram transition calculations in logic blocks wherein at least some of the logic blocks used to implement the binary frequency shift keying operation are also used in the quaternary frequency shift keying operation.
Still yet another embodiment of the present invention is a receiver comprising an A/D converter adapted to sample at a rate such that there are multiple samples for each transmitted symbol; a history convolver unit associated with a maximum likelihood sequence detector, the history convolver unit adapted to produce an estimate of an ideal sample value; and a sample selection unit including a timing correction unit creating early, on-time, and late error signals using early, on-time, and late samples along with the estimate of an ideal sample value. The timing correction unit using the early on-time and late error signals to determine whether to advance, retreat, or not modify the sample slot used in the demodulation.